The Microwave Interferometric Reflectometer (MIR) is a new airborne reflectometer being developed at Catalonia Polytechnic University (UPC) that uses Global Navigation Satellite Systems (GNSS) to retrieve mainly altitude over sea, but also soil, ice and sea parameters such as soil moisture, vegetation water content, sea surface roughness and wind speed. It uses two frequency bands and two different beams per band in order to improve the sensor capabilities. These beams point to the estimated specular reflection area and to the desired satellites considering the position of the platform.